In offshore petroleum production, drilling vessels are used on the sea surface to drill wells in the seabed. A wellhead is installed on the seabed and is sealingly connected to the well casing and provided with the necessary control and contingency structures including a valve assembly (BOP) and connecting devices to a riser which connects the wellhead to the drilling vessel at the sea surface. The riser is used in connection with the return and circulation of drilling fluids from the well and is normally filled with drilling fluid in the drilling phase which both lubricates the bit and allows return of cuttings in the circulating fluid. The riser is in addition equipped with longitudinal, externally located smaller pipes (choke, kill and optionally booster line) which are connected to a valve manifold on the drilling vessel. These pipes are used, inter alia, in connection with the replacement of fluid in the riser and for adding more drilling fluid in the well to improve well control if the well pressure rises by an intolerable amount whereupon it may be necessary to close the valve assembly (BOP) that is connected sealingly to the wellhead underneath and the riser above in order to prevent undesirable outflow of drilling and well fluids.
In order to commence drilling a drill string is lowered into the riser, which string may consist of sections of pipe and which at its lower end has a bottom hole assembly including drill collars and a drill bit. There are various ways of driving the bit known in the art for example using a top drive motor on the drilling derrick but typically the circulating drilling fluid may be used with a borehole (“mud” or Moineau) motor to drive the bit taking account of the length of string necessary to reach the required greater depths of current wells.
During deep water drilling and production operations, there may arise circumstances where it becomes necessary to disconnect the marine riser from the wellhead. Those in the field will understand the difficulties of effecting a safe and environmentally sound disconnect of the riser in view of the need to displace drilling fluids or completion fluids as a preliminary step in such a riser disconnect. In particular the operation to displace the drilling or completion fluid to seawater requires a careful and methodical approach yet in practice the time available may be limited to complete such an operation e.g. due to an approaching storm.
In an emergency shut-down and disconnect, the riser may be suspended open-ended which could have an environmental impact if well fluids or working fluids leak from the open end of the riser. Consideration has to be given to clearing the riser of drilling or completion fluids etc. without causing adverse environmental consequences, without loss of the expensive fluids into the environment, and allowing, when necessary, subsequent safe topside dismantling of the riser components without contaminating the deck or drill floor. If spillage of drilling fluids takes place during subsea unlatching of the riser or in a subsequent riser pull out and lay down operation, then it is necessary to follow this with a clean up operation which is both expensive and time-consuming. If the rig had to be secured rapidly due to an approaching storm, then for safety reasons a clean up would not be possible until after the storm passes, in which event the clean up operation would also delay re-start of normal operations.
Current practice would be to pump the displacement fluid via lines in the well head (kill, choke or booster lines normally present in the wellhead structure for other purposes can be utilised) to displace the riser fluid upwards to surface for recovery and storage.
A drilling fluid is typically of an inverse emulsion character where the continuous (external) phase is oil-based and the discontinuous (internal) or dispersed phase is aqueous-based. Special purpose surface active additives stabilise the fluid for use so that it retains its intended design characteristics and properties. Other fluids used within the riser include completion fluids which have a different chemical composition, typically being brines such as chlorides, bromides and formates. Such a drilling or completion fluid will be referred to hereinafter as a “riser fluid” for ease of discussion in the context of this disclosure. The displacement fluid may be any environmentally benign fluid such as seawater or brine or an equivalent.
Typically, during displacement of the riser fluid prior to disconnect, the introduced displacement fluid, e.g. seawater, meets the riser fluid at an interface which in practice is not clearly defined, and a fair amount of mingling of the dissimilar fluids (perhaps 10% or more of the riser volume) takes place during the riser displacement operation. As a consequence, operators have been faced with recovering the fluids and confronting the reality that a fair proportion of the riser fluid will be contaminated with seawater. From another standpoint inevitably a volume of seawater is captured with riser fluid contamination and this cannot be discharged overboard, and must be processed. Either way, an operator has to process a significant volume of fluids which may be separated but only after input of energy and resources. This may entail storing the mingled fluids or partially separated fluids for shipment to a shore installation for final processing treatment. This problem is particularly acute at deep water sites where the riser volume may be as much as 2,000 barrels of fluid or more.
In the past, attempts to address the mingling problem have included introduction of a plug or pill of a gelled viscous material that is insoluble or immiscible in the respective fluids to act as a physical barrier to mingling.
European patent number EP 1 937 930 describes a method and device for preventing mixing of fluids in a riser.